Products of nanotechnology, such as semiconductor and other electronic devices, are manufactured on the surface of substrates. Any surface imperfections with dimensions greater than feature sizes of device structures may adversely affect device functionality and lead to device operation failures. Therefore, a great deal of attention in nanotechnologies is paid to preventing contamination and/or damage of the surface of substrates or workpieces, in general meaning. Methods and tools for analyzing surfaces of workpieces found multiple critical applications in nanotechnologies and other areas concerned of surface quality. Prime parameters which differentiate surface analyzers are throughput, sensitivity, defect resolution and cost.
High performance surface analyzers have been developed for inspection of semiconductor wafers. The prime inspection parameters include surface roughness, localized defects, large area defects and scratches. The surface analysis is based on a systematic scanning of a wafer surface with a laser beam. Laser beam radiation scattered on surface imperfections is collected and quantified. Judging on characteristics of the scattered laser radiation, such as intensity of scattered radiation, a nature of surface defects and their dimensions are determined. Also, locations of identified defects are recorded and reported.
A scanning laser beam illuminates an analyzed surface with a beam spot. The throughput of surface analyzers is proportional to the speed of the beam spot movement on the analyzed surface. The “spiral” laser scanning is the most efficient option applied in wafer inspection. It is conventionally implemented with a stationary systems of laser illumination and stationary systems of detection of scattered and, optionally, reflected radiation. A wafer is spun and simultaneously moved along a radial direction so that the laser beam spot on the wafer surface follows a path of an Archimedes spiral. Spiral scanning was first applied for inspecting wafer decades ago (see Altendorfer, H. and Kren, G. “Unpatterned surface inspection for next-generation devices”, Solid State Technology. 1996, Vol. 39, Issue 8, pp. 93-96) and since then it became the mainstream approach that has been providing the highest inspection throughput (Tuyen K. Tran “Defect Characterization and Metrology” in Ma, Z. and Seiler, D. G. “Metrology and Diagnostic Techniques for Nanoelectronics”, Singapore: Pan Stanford (2017), pp. 592-607).
The concept of spiral scanning is illustrated in FIG. 1. A semiconductor wafer 101 is placed on a rotary table that spins the wafer about its center 104. As an example, the wafer spins in the clockwise direction 106 but any direction of spinning may be applied. A laser beam provided by a stationary source illuminates the wafer 101 with a laser spot. The rotary table is mounted on a linear actuator that moves the rotary table with the wafer 101 in a direction 103 perpendicular to the axis of wafer spinning. An inspection starts with the laser spot located at the center 104 of the wafer 101 and then proceeds until the laser spot on the wafer surface reaches the location 102 at the edge of the wafer 101. The spinning and linear motion of the wafer are synchronized such that the scanning goes track-by-track along the spiral path 105 on the surface of the wafer 101. The boundaries 107 of the scanning tracks correspond to the width of the laser beam spot. Instantaneous laser spots 108 are shown on the last two circles of the spiral path 105. If the distances 109 and 110 between adjacent scanning tracks are not greater than the width of beam spots 108, the entire surface of the wafer 101 is scanned without gaps.
Examples of spiral scanning implementation may be found in U.S. Pat. No. 4,314,763 “Defect detection system” by Steigmeier, et al.; U.S. Pat. No. 6,201,601 “Sample inspection system” by Vaez-Iravani, et al.; U.S. Pat. No. 6,606,153 “Process and assembly for non-destructive surface inspections” by Marxer, et all.; U.S. Pat. No. 7,791,721 “Surface inspection with variable digital filtering” by Takahashi, et al.; U.S. Pat. No. 8,885,158 “Surface scanning inspection system with adjustable scan pitch” by Wolters, et al.; U.S. Pat. No. 8,891,079 “Wafer inspection” by Zhao, et al.; U.S. Pat. No. 8,934,091 “Monitoring incident beam position in a wafer inspection system” by Reich, et al.; U.S. Pat. No. 9,255,891 “Inspection beam shaping for improved detection sensitivity” by Wolters, et al.; U.S. Patent Application No. 20180038803 “Surface Defect Inspection With Large Particle Monitoring And Laser Power Control” by Cui, et al. Wafer analyzers with spiral scanning achieve high throughput through the fast rate of wafer spinning. It is important that the linear motion in spiral scanning is slow and continuous so there are no detrimental impacts of inertia on the accuracy of wafer positioning during surface analysis.
Semiconductor wafers are precisely shaped disks with axial symmetry. However, in practice, the rate of wafer spinning is limited because of inevitable mass imbalances associated with inconsistency of wafer positioning on rotary tables, wafer warpage, etc. Methods and tools for surface analysis with high sensitivity and high resolution apply narrow laser beams. Therefore, scanning an entire wafer surface may only be accomplished with a significant number of spinning revolutions implying a notable time consumption. This major limitation of spiral scanning may become more critical with industry transition to large diameter 450 mm wafers. Several solutions have been developed in the prior art to enhance scanning efficiency of wafer surface analyzers.
For example, U.S. Pat. No. 5,712,701 “Surface inspection system and method of inspecting surface of workpiece” by Clementi, et al. describes a wafer surface analyzer with a system that deflects a laser beam within a narrow scan path of <0.1 radians along the surface of the wafer. In a combination with the conventional spiral scanning, the added component of scanning by high frequency beam deflection enhances wafer inspection throughput. As a drawback, this solution complicates the analyzer, reduces its serviceability and calibration, increased its dimensions and cost of ownership. Besides, the laser beam deflection leads to a change of the angle of laser beam incidence on the wafer surface that may cause some inconsistency in defect detection.
Spiral scanning may be accelerated by adjusting the pitch of scanning during a wafer inspection according to a desired sensitivity to defect detection and throughput—see U.S. Pat. No. 8,885,158 “Surface scanning inspection system with adjustable scan pitch” and U.S. Pat. No. 9,116,132 “Surface scanning inspection system with independently adjustable scan pitch” by Wolters, et al. This approach may not provide throughput benefits when a high resolution surface analysis is required. In overall, there is a need for further improving throughput of wafer surface analyzers.
High throughput surface analyzers with spiral scanning may be effectively applied to inspecting light and axially symmetrical objects such as semiconductor wafers. However, high spinning speed may not be practically implemented for heavy and/or unsymmetrically shaped workpieces. Another drawback of analyzers with spiral scanning is their large dimensions. Rotary tables must have means to firmly hold analyzed workpieces. They also need to be combined with moving stages capable of repositioning a characterized object at distances not less than a half of the lateral dimension of the object. Therefore, surface analyzers with spiral scanning are usually stand-along large tools which have high cost of ownership and cannot be integrated into or within production equipment. Yet another problem of conventional analyzers of technological surfaces is a complexity of their optical systems and unique precision required at their servicing.
The present invention has an objective to provide an analyzer of technological surfaces capable of high precision and resolution and having advantages over the state of the art in (i) attaining higher throughput and efficiency; (ii) analyzing workpieces of arbitrary shapes and mass; (iii) compactness, portability and ability to be integrated into manufacturing or research equipment for in-situ analysis and process or equipment diagnosis; (iv) higher robustness, serviceability and lower cost.